1. Field of the Invention
The present invention relates to a power source for an electronically tuned tuner for receiving television signals and other broadcast signals.
2. Description of the Prior Art
FIG. 4 shows an example of a conventionally known power source which comprises a horizontal deflection circuit 1, a fly-back transformer 2, a tuning voltage generating circuit 3, an electronic tuning front end 4, a resistor 5, and a Zener diode 6.
When television signals are received by the electronic front end 4, it is necessary to provide a relatively high power source voltage of about 30 volts in order to produce a tuning voltage V.sub.T because the frequency band of UHF is wide, namely, 470-770 MHz. In a television receiver in which AC is used as a power source, the power source voltage can be easily obtained. In a television receiver in which a DC power source is used, on the contrary, there is provided, generally, only a DC power source of 12 volts, 9 volts, or the like. Therefore, for example as shown in FIG. 4, a power source voltage of about 40 volts is formed by the fly-back transformer 2 and this voltage is stabilized by the Zener diode 6 so as to be used as a source voltage.
However, since the source voltage is always supplied to the tuning voltage generating circuit 3, it is required to operate the horizontal deflection circuit 1 to obtain the source voltage from the fly-back transformer 2 even in the case of reception of FM broadcasting or medium wave broadcasting signals where only a low tuning voltage is required. This results in a disadvantageous consumption of power and in shortened life of parts because a large amount of power is handled in the horizontal deflection circuit 1. This further results in generation of noise because high harmonic waves exert influence over AM broadcasting band. Further, when other than television signals are received, the synchronizing signals disappear and therefore there is a possibility of lowered voltage from the fly-back transformer 2 or of generation of abnormally high voltage in the case that the horizontal AFC frequency is shifted due to time aging or any other reason. Accordingly, it has been considered to separately provide a dedicated DC-DC converter. Although such a dedicated DC-DC converter makes it possible in fact to stop the horizontal deflection circuit 1 in the case other than reception of television signals, there are unavoidable disadvantages in view of cost and space for mounting the converter.